What have been the principal causes of mass migration since WWII? What are some of the challenges presented by these migrations to host countries?

c

explanation:

roosevelt believed that in light of the country's recent military successes, it was unnecessary to use force to achieve foreign policy goals, so long as the military could threaten force. this belief, and his strategy of “speaking softly and carrying a big stick,” shaped much of roosevelt's foreign policy.

i think it’s a i hope that right : )